What does the line above a letter represent? 
The image above is from Crash Course and I saw the equation the velocity is Delta.
What does the line above acceleration represent?
 A: The bar over the $'a'$ means average acceleration. It's not a typo in the video. A bar above $'v'$ would denote average velocity.
A bar above any quantity indicates that it is the average value of that quantity. 
A: Usually, a bar over a symbol means its average. Also, one can use the symbols $\left< - \right>$ can be used to denote the same thing.
Usually, this average is a time average, that is
$$\left< s \right> = \frac{1}{t_1-t_0}\int\limits_{t_0}^{t_1} s(t)\textrm{d}t$$
where either $t_0 \rightarrow -\infty$ and $t_1 \rightarrow +\infty$, or $t_0$ is the start of the experiment and $t_1$ its end.
In your case, assuming the experiment starts at $t=0$ and lasts $\Delta t$, it gives
$$\left< a \right> = \frac{1}{\Delta t}\int\limits_0^{\Delta t} a(t)\textrm{d}t = \frac{1}{\Delta t} \int\limits_0^{\Delta t} \frac{\textrm{d}v}{\textrm{d}t}\textrm{d}t = \frac{v(\Delta t) - v(0)}{\Delta t} = \frac{\Delta v}{\Delta t}$$
Notice the use of the symbol $\Delta$ to denote a variation of a function. Here for example $\Delta v$ means the variation of the function $v$ of variable $t$. You could also encounter the symbol $\textrm{d}v$: both $\Delta$ and $\textrm{d}$ mean the same thing (ie. a variation of something), however $\textrm{d}$ means that it is an infinitesimal variation. From a mathematical point of view, this is complete nonsense: what is a difference between a big, a normal, and a small variation ? And what do big, small mean ? In fact, the difference is that $\textrm{d}$ implies implicitly a limit, while $\Delta$ is just a normal quantity. Then whenever you see a $\textrm{d}s$ instead of a $\Delta s$, you should interpret it as when the variation of $s$ tend toward zero, then ... You can now understand the notation $\frac{\textrm{d}}{\textrm{d}t}$ to denote the temporal derivative. Let's consider a function $f(t)$, then
$$f'(t) = \lim_{\textrm{d}t\to 0} \frac{f(t+\textrm{d}t) - f(t)}{\textrm{d}t} = \frac{\textrm{d}f}{\textrm{d}t}$$
Similarly, when you are referring to a small quantity of something, and not a variation of something, you should use the symbol $\delta$ (which implies a limit) and no symbol at all when the quantity isn't small. For example, if you consider the heat transferred during a whole experience, you could call it $Q$, while during a infinitesimal transformation it should be $\delta Q$
